A major problem encountered in the metal fabricating industry is the need to machine holes, slots, grooves and the like in a workpiece in accordance with a set of drawings of said part. These drawings always give dimensioned location of holes, grooves, etc. in relationship to the zero point of two edges, typically referred to as the zero X & Y points. Machine tools, such as milling machines generally include a chuck or collet for holding the cutting tool and a workpiece holder such as a vise which is rigidly affixed to a work table that can be moved in both the X & Y axis by means of manual dials or motorized drives either under manual or computer control.
For example, if a hole is to be drilled in a workpiece in the precise location indicated on an accompanying drawing, the operator must first locate the zero X and Y location of the workpiece. This X & Y zero point corresponds to the exact center of the drill bit held in a chuck or collet. Once the X & Y zero location has been located, the workpiece is then moved the exact distance indicated on accompanying print so that the hole can be drilled in the correct location.
Movement distance after obtaining the X & Y zero points are calculated by the use of graduated dial collars on the X & Y turn handles or through the use of the more common digital travel display. The digital dial collars or digital display are zeroed out when each respective edge is located. The table is then moved the desired distance in both the X & Y axis until the cutting tool is properly located on the workpiece to accomplish the desired machining operation.
Various types of electrical and non-electrical edge finders are currently being used to locate the above stated X & Y zero points. Both the electrical and non-electrical types must be mounted in the cutting tool location and therefore the cutting tool must first be removed, then the edge finder is mounted in the cutter position. Assuming we are using an electric type edge finder as illustrated in U.S. Pat. No. 4,386,344, we now bring the quill downward until the tip of the edge finder s just to the outside of the workpiece and slightly below the top surface on the X axis. Next the table holding the workpiece is moved towards the edge finder until we just make contact. The contact is indicated by the light turning on.
The dial collar or digital display is then reset to zero and the edge locating process is repeated to verify the accuracy of the first operation. We verify the accuracy by seeing that the dial collar or digital display is on zero point when the edge finder light turns on. If the displays are not on zero, we must again reset the dial collar or digital display to zero and repeat edge finding process until we have verification.
Since the current edge finding process relies on visual judgement only, the operator must move the table slowly to be sure there is no overtravel after contact is made. Once the edge location has been verified, the edge finder is moved above the workpiece and the table is moved another distance equivalent to 1/2 of the diameter of the edge finder probe. The dial collar or digital display is again reset to zero and now we have the true center point of the cutting tool aligned to the exact edge of workpiece.
These steps must now be repeated for the Y axis. After locating the X & Y zero position, the edge finder is removed and the cutting tool put back in. Now the machining process can begin.
As can be seen from the above description, properly locating the zero X & Y reference edge of a workpiece is quite time consuming and the operator must take care in judging the exact time of probe contact with the workpiece and be sure to not overtravel. While the electrical type is somewhat quicker to use in terms of locating a precise edge, they depend on an electrically conductive path between workpiece and edge finder and cannot therefore be used on any non-conductive material such as plastics.
The non-electrical type require that the machine spindle turns while the operator looks for a slight axial disalignment to occur at the tip of the edge finder. The non-electrical type is therefore somewhat slower to use and requires more care than the electrical type.
As can be seen, both of the above indicator types require a three step process to find the true zero point of any edge. After the edge finder first locates the edge of the workpiece, the digital display, or equivalent, is set to zero. The edge finding process is then repeated to verify accuracy and then the table is moved half the distance of the edge finder in order to locate the true zero reference point.
In both of the above edge finder methods, the workpiece must approach the edge finder in a slow and careful manner to prevent overtravel which, if excessive, will damage the edge finder tool.
Also, the edge can only be located when moving the table in one direction. That is while the workpiece is being moved towards the edge finder.
As can be easily understood from the above description, both of the methods currently in use have some major problems.
1. They are very time consuming to use as existing cutting tool must be removed so that edge finder can be mounted in that location and after locating the reference edges, the edge finder must again be replaced by the cutting tool before any work can be done.
2. The actual edge finding process requires careful and slow approach to the workpiece when coming into contact with edge finder.
3. The edge finding process should be repeated at least twice for each edge in order to assure accuracy.
4. After completing step three above, the workpiece must still be moved to compensate for the radius of the edge finder.
5. Each time the cutting tool is removed the Z reference point (depth of cut) is lost and therefore must be reindicated after edge finder is removed and the cutting tool is re-installed.
6. The accuracy of both methods is highly dependent on operator skill, judgement and hand to eye coordination.